Be sure that there is vacuum in the plastic line going to the modulator. Try sucking on the modulator to be sure that has a tight diaphram.

If all is well then I would try adjusting the modulator. I am interpreting your condition to be hard shifting. I get a bit confused because this can actually be the opposite of "shuddering". I describe shuddering as a multiple shift where one gear change appears to be accomplished through multiple steps. For a harsh single shift I would start with a full turn counterclockwise.

If there is a shudder as I described it then you will probably need to make the shift firmer or possibly earlier.